Figure 1.

Focus simulation control menu. “3D Step Size” sets layer thickness with value “1”
for 0.25μm. “3D Focus Layers” sets the total number of layers with value “40” for
40 layers on both side of autofocus plane. This setting, 3D Step Size: 1 and 3D Focus
Layers: 40, results 0.25 X (40+1+40), almost 20μm as total thickness to scan.